Deleted Replies

I understand that some content should be deleted either by an OP or mods if the need arises. I am curious about something though. If someone replies to something you post and the entire post gets deleted, you can still see the response in the notification screen. However, if the specific reply gets deleted or removed you are unable to see the response anywhere.

So I guess I an curious about the differences in the two cases as to why you can see or cannot see in the situations. Then if there is a way to see the latter since was curious about a reply someone gave me. Was it deleted by the OP or was it removed? Is there a way one can still see the reply?